An underwater pipeline is normally assembled on board a laying vessel, and laid on the bed of a body of water as it is assembled. One commonly used type of laying vessel comprises a substantially horizontal, on-board assembly line; and a curved lay ramp that guides part of the pipeline as it is lowered onto the bed. This laying method is known as S-laying, because of the shape of the pipeline between the laying vessel and the bed.
On another commonly used type of laying vessel, the pipeline is assembled in a substantially vertical lay tower and released substantially vertically. This laying method is known as J-laying, because of the shape of the pipeline between the laying vessel and the bed, and is preferable to S-laying when working in deep water.
Substantially two situations make it necessary to abandon the pipeline on the bed: bad weather conditions making laying work dangerous; and completion of the pipeline.
The pipeline must be recovered off the bed to resume laying and assembly work interrupted by bad weather.
Methods of abandoning and/or recovering underwater pipelines comprise shutting down assembly of the pipeline; connecting the free end of the pipeline to a hoisting assembly comprising at least one winch on the laying vessel, at least one rope, and a connecting device for connecting the rope to the pipeline; and abandoning/recovering the pipeline by winching out/up the rope. Documents EP 1,850,043 A2; U.S. 2007/0177944; WO 2009/002142; and WO 2009/082191 describe various abandoning and/or recovery methods, which employ hoisting assemblies comprising at least one winch on board the laying vessel, and one rope. Some hoisting assemblies employ a first and second winch for synchronously operating a first and second rope to share the load exchanged between the pipeline and the laying vessel.
The load between the laying vessel and the pipeline varies between a maximum and minimum, and depends on the length of pipeline raised off the bed. When abandoning the pipeline, load is maximum at the initial stage, when a long portion of the pipeline is raised off the bed. And, conversely, when recovering the pipeline, load is maximum at the final stage, when, again, a long portion of the pipeline is raised off the bed.
The load between the pipeline and the laying vessel, in fact, is a function of the weight per linear meter of the pipeline, and the length of pipeline raised off the bed.
Whether it comprises one or more winches, the hoisting assembly must have a total capacity greater than the actually short-lived maximum load between the pipeline and the laying vessel. Which means the laying vessel as a whole must be equipped with a hoisting assembly capable of hoisting more than the maximum load exchangeable between the pipeline and the vessel. Deepwater laying vessels, normally equipped with a J-lay tower, must therefore be equipped with hoisting assemblies of over 1500-ton capacity when working with extra-large-diameter pipelines.
The first and second rope are normally metal-stranded, substantially the same length as or longer than the depth of the body of water, and of a diameter consistent with the maximum load for which they are designed. More specifically, the first and second rope may be as long as 1500 meters, with diameters ranging between 0.08-0.25 meters, which makes them extremely bulky and expensive.
A major problem posed by hoisting devices comprising a first and second winch and a first and second rope is the tendency of the ropes to become entangled as they are wound up.
That is, once the pipeline is abandoned on the bed of the body of water, the first and second rope are disconnected and rewound onto the laying vessel. Given the depth of the body of water, rewinding the ropes takes some time, during which they invariably become crossed and entangled, also in view of the fact that the pipeline is abandoned in severe weather and surge conditions.
Antirotation ropes are available on the market, but ropes capable of lifting and lowering exceptional loads are normally twisted-strand types which, under their own weight, rotate about their longitudinal axis as they are rewound. And, since the ends of the ropes are free, this rotation causes the ropes to swing, thus increasing the danger of them becoming entangled.
Entangling of the first and second rope may result in incomplete or slow rewinding, and, in some cases, in considerable downtime to disentangle them.